Transcriptional repression by ApiAP2 factors is central to chronic toxoplasmosis

Abstract

Bradyzoite differentiation is marked by major changes in gene expression resulting in a parasite that expresses a new repertoire of surface antigens hidden inside a modified parasitophorous vacuole called the tissue cyst. The factors that control this important life cycle transition are not well understood. Here we describe an important Toxoplasma transcriptional repressor mechanism controlling bradyzoite differentiation that operates exclusively in the tachyzoite stage. The ApiAP2 factor, AP2IV-4, is a nuclear factor dynamically expressed in late S phase through mitosis/cytokinesis of the tachyzoite cell cycle. Remarkably, deletion of the AP2IV-4 locus resulted in the increased expression of bradyzoite mRNAs in replicating tachyzoites, and in two different genetic lineages we confirmed the misexpression of tissue cyst wall components (e.g. BPK1, MCP4, CST1) and the bradyzoite surface antigen SRS9 in the tachyzoite stage. In the murine animal model, the loss of AP2IV-4 had profound biological consequences. Type II prugniaud strain parasites lacking AP2IV-4 were unable to form tissue cysts in brain tissue and the absence of this factor also recruited a potent immune response characterized by increases inflammatory monocytes, IFN-γ and higher numbers of both CD8+ and CD4+ T-cells. Altogether, these results indicate that suppression of bradyzoite antigens by AP2IV-4 during acute infection is required for Toxoplasma to establish a chronic infection in the immune-competent host.

Toxoplasma gondii AP2IX-4 Regulates Gene Expression during Bradyzoite Development

“…In a separate study, we have identified a bradyzoite transcriptional repressor in the tachyzoite that fits this classic pattern (40). …”

Brains and Brawn: Toxoplasma Infections of the Central Nervous System and Skeletal Muscle

“…For example, Api2IV-4 normally expressed by tachyzoites, inhibits bradyzoite gene expression. Parasites in which Api2IV-4 is deleted increase their expression of bradyzoite antigens, leading to enhanced immune responses and a complete absence of cysts in the brain [26]. This suggests that turning off bradyzoite antigen expression is as important to chronic infection as turning it on.…”

Toxoplasma gondiiAP2IX-4 regulates gene expression during bradyzoite development

“…4), it is likely that the lack of synchronization in the microarray study underestimated the magnitude and number of genes This pattern of gene expression in AP2IX-4 knockout parasites is not the classic profile of a transcriptional repressor, where mis-expression of bradyzoite genes in the tachyzoite would be expected. In a separate study, we have identified a bradyzoite transcriptional repressor in the tachyzoite that fits this classic pattern (38).…”

“…In addition, the unbalancing of gene expression during bradyzoite development caused by the loss of AP2IX-4 could cause problems with either tissue-specific formation of the tissue cyst or in achieving the proper escape from the host immune system. Preventing the pre-arming of the immune system against the parasite surviving to form the tissue cyst is a confirmed function for AP2IV-4 (38) and likely has driven the evolution of AP2IX-4 function. Validating this function will require a complete dissection of the AP2IX-4 transcriptional mechanism and its molecular partners during bradyzoite development in vitro and in vivo.…”

“…AP2IX-9 represses the expression of bradyzoite mRNAs; overexpression of AP2IX-9 promotes tachyzoite growth during stress and prevents formation of tissue cysts (21). AP2IV-4 is a cell cycle-regulated factor with peak expression in the tachyzoite S/M phase that also suppresses bradyzoite gene expression (38). In contrast, AP2IV-3 is an activator of bradyzoite gene expression; a knockout reduces tissue cyst formation and its overexpression increases tissue cyst formation (24).…”